Conventionally, various video image expressions such as gain adjustment of a video image signal and enhancement of a contrast are tried (for example, Patent Document 1).
When a powerful video image expression is tried, processing for enhancing screen luminance is given because a brightness feeling of a screen is required. In this point, Patent Document 2 describes that, conventionally, a video image display is performed by increasing a gain for a pixel that is in a low to intermediate region of gamma correction properties for making a brightness feeling of a display video image and by decreasing a gain for a high region pixel to prevent saturation in a bright pixel at the same time.
Meanwhile, such gain control has a problem that a gain becomes smaller and luminance becomes lower in most pixels for a totally bright video image, and therefore the gain control is not applied uniformly to all input video images but control being different for each of video image properties is tried to be applied.
Patent Document 3 describes one of these techniques. In this document, after the amplitude peak in RGB signals of input video image signals is detected, in a gamma circuit for a video image signal without containing a high peak component, processing for small amplification of a signal in a high region is performed along with linear amplification of a signal in or below an intermediate region of brightness (gamma properties are shown in (b) of FIG. 23), and for a video image signal containing a high peak component, a video image signal is output as it is after releasing a gamma operation (gamma properties are shown in (a) of FIG. 23) to prevent clipped whites (blown out highlights) at the time of large amplitude.
Additionally, in Patent Documents 4 and 5, an average luminance (hereinafter, referred to as “APL”) of an input video image signal is detected and the lower the detected APL, the greater an amplification degree of a signal in or below an intermediate region of brightness is made. (in FIG. 24, the lower APL makes gain control in order of (b)→(c)→(d)), while when the APL is higher than a certain value, a video image signal is output as it is without performing the gain control ((a) in FIG. 24) to prevent saturation on the white side.
In the above Patent Documents 3 to 5, an operation is such that when a video image characteristic amount meets predetermined conditions, an input signal is amplified all the time and when a video image characteristic amount does not meet predetermined conditions, an input signal is not amplified all the time.
That is, in Patent Document 3, when an APL as a video image characteristic amount meets conditions that the amplitude peaks of RGB signals don't contain a high peak component, a signal is amplified all the time and in the case of not meeting the conditions (in the case where the amplified peaks of RGB signals contain a high peak component), a video image signal is not amplified all the time.
Similarly, in Patent Documents 4 and 5, in the case of meeting conditions that an APL as a video image characteristic amount is a certain level or lower, an amplification degree of a signal with brightness being an intermediate value or less (certain luminance value or less) is increased in proportion to a low level of an APL (amplifying all the time) and in the case of not meeting conditions that an APL is a certain level or lower (in the case where a APL is higher than a certain value), the gain control is not performed and a video image signal is not amplified.
As described above, a video image characteristic amount such as a peak of an input video image signal and an APL is detected to control a gain of an input video image signal depending on the detected result and thereby not only clipped whites are prevented but also a higher contrast is attempted, while there may be also a preferable case that a video image signal with a luminance that is a certain level or lower is boosted to make a brightness feeling, for example, when a dark video image signal is contained at a little bit high rate even in the case of not meeting conditions that an APL is a certain level or lower because whether or not amplification processing of a video image signal is performed is determined depending on specific conditions that a video image characteristic amount meets.
Additionally, there is also a case that amplification of a video image signal may be better to be prevented from performing in consideration of decreasing luminance in a high luminance part in a case where a pixel with high luminance is contained at a little bit high rate such as a case of giving priority to prevention of saturation on the white side, even when an APL is significantly low.
Furthermore, for a video image having a small level of a peak and an APL, deepness of a black level may be sacrificed at any cost because processing for boosting a video image signal with a brightness that is an intermediate value or lower is performed.
Since the video image expression is performed by performing light modulation of light source luminance in addition to processing of a video image signal in a video image display apparatus such as a liquid crystal display apparatus that displays a video image by irradiating light from the back side of a display screen, more complicated video image expressions are allowed with an appropriate combination thereof.
Patent Documents 6 to 9 describe a liquid crystal display apparatus enriching the video image expression especially on the black side by attempting to combine processing of a video image signal with light modulation of light source luminance.
In these Documents, it is described that the luminance of a light source is lowered to provide deepness of a black level not being capable of the expression only with a normal video image display and a video image signal is amplified to prevent from lowering a luminance level of the whole video image.
Describing in more detail about Patent Document 6, after a histogram of an input video image signal is analyzed to select luminance of a light source capable of the best expressing histogram thereof, a video image signal is amplified to compensate the lowered luminance in a case where the light source is lowered. Such amplification rate is calculated by a formula of (light source luminance at a maximum light emission/light source luminance in the case of being lowered)1/γ, which allows a video image signal to be amplified in a case where a light source is lowered.
As such, technology described in Patent Documents 6 to 9 provides processing of which the luminance of a light source at a maximum light emission does not allow a video image signal to be amplified and the lowered luminance of a light source allows a video image signal to be amplified. That is, there is also a case that a video image signal is not or is amplified despite conditions of a video image characteristic amount of a video image signal.
Additionally, Patent Document 10 describes that an APL and a maximum/minimum value of an input video image signal is detected to enlarge an input video image signal to a dynamic range and the luminance at the time of display of video images deviated by enlarging a video image signal is compensated by adjusting light emission luminance of a light source.
It can be described by a technology of the patent document that a video image signal is not amplified all the time in the case of meeting conditions in which maximum luminance and minimum luminance as a video image characteristic amount of an input video image signal correspond with a dynamic range and a video image signal is amplified all the time in the case of not meeting a condition in which maximum luminance and minimum luminance correspond with a dynamic range (in a case where maximum luminance and minimum luminance do not correspond with a dynamic range).
Additionally, a rate emphasizing the above black level, a rate emphasizing the white, the opposite thereof, a degree emphasizing power saving and the like do not have all video images at the same degree of emphasis but have different situations depending on respective video image characteristics. Regarding this point, for example, in Patent Document 11, light source luminance control properties of a liquid crystal display are changed to achieve to obtain the desired display luminance and to lower power consumption according to an image quality mode such as a dynamic mode, a normal mode, a movie mode and a game mode. Furthermore, in Patent Document 12, a mode that indicates to which a priority is given, the image quality or power saving, is provided to change the degree of emphasis of power saving for backlight according to this mode.    [Patent Document 1] Japanese Laid-Open Patent Publication No. 6-62277    [Patent Document 2] Japanese Laid-Open Patent Publication No. 2006-101363    [Patent Document 3] Japanese Laid-Open Patent Publication No. 6-350874    [Patent Document 4] Japanese Laid-Open Patent Publication No. 2003-167544    [Patent Document 5] Japanese Laid-Open Patent Publication No. 2003-309741    [Patent Document 6] US 2006/0274026    [Patent Document 7] Japanese Laid-Open Patent Publication No. 2006-276677    [Patent Document 8] Japanese Laid-Open Patent Publication No. 2006-267995    [Patent Document 9] Japanese Laid-Open Patent Publication No. 2007-36728    [Patent Document 10] Japanese Laid-Open Patent Publication No. 2001-27890    [Patent Document 11] Japanese Laid-Open Patent Publication No. 2007-140436    [Patent Document 12] Japanese Laid-Open Patent Publication No. 2007-219477